In the construction of exhaust gas purification catalysts for automobiles and the like, platinum, palladium or rhodium, which are catalytic metals, plus auxiliary catalyst that increases the catalytic effect thereof, are supported on a catalyst support such as alumina, cordierite or the like, for example. A characteristic of said auxiliary catalyst material is that it absorbs oxygen in an oxidizing atmosphere and releases oxygen in a reducing atmosphere. Auxiliary catalyst materials having such a characteristic efficiently purify harmful components in exhaust gas, namely hydrocarbons, carbon monoxide and nitrogen oxides, and are therefore used to maintain an optimum air/fuel ratio.
The efficiency of exhaust gas purification by exhaust gas purification catalyst is usually proportional to the area of contact between the catalyst metal active species and the exhaust gas. Maintaining said optimum air/fuel ratio is also important, and so a high reduction rate must be maintained for the auxiliary catalyst oxygen absorption/release. Specifically, the tightening of exhaust gas regulations brings demand for an auxiliary catalyst material that has high heat resistance and, at the same time, exhibits high redox ability even when the catalyst temperature is low, as when starting a cold engine, for example.
Several complex oxides that exhibit redox ability at and below 400° C. have already been proposed. For example, patent document 1 proposes a CeZrBi complex oxide that exhibits high redox ability at and below 300° C. However, when this complex oxide is exposed to reducing conditions at or above 700° C., the bismuth oxide is reduced to metallic bismuth, and vaporizes, and so on repeated oxidation and reduction, the bismuth component in the complex oxide is depleted and the redox characteristic deteriorates. Practical use in automobile catalysts, which undergo repeated redox at high temperature over long periods, is therefore difficult. Patent documents 2-4 propose complex oxides comprising CeZrBi plus Ba, Ag or Pt, respectively, where the fourth component is added to improve heat resistance or phase stability. However, on exposure to a reducing atmosphere at high temperature, vaporization of the bismuth component is cause for concern.
Patent documents 5-8 and the like propose the addition of a rare earth metal element or silicon as stabilizer, in order to improve the heat resistance etc., of cerium oxide. Proposed in these documents are various complex oxides with excellent heat resistance at high temperatures, and excellent specific surface area maintenance, according to the BET method.
Nevertheless, specifically, there is no known complex oxide containing combinations of cerium, silicon, rare earth metal elements other than cerium, and other elements, that exhibits excellent heat resistance and adequate reduction rate even at low temperature.